Conventional boiling water reactor fuel bundle assemblies utilize one or more water rods extending upwardly through the bundle to provide a source of coolant/moderator to maintain a more uniform distribution of power throughout the bundle. For convenience, reference will be made to water rods in the plural, recognizing that a single water rod bundle is also well known in the art. Typically, the water rods are closed at both ends by end plugs received in upper and lower tie plates of the bundle assembly. Side entry and exit holes are formed in the lower and upper portions, respectively, of each rod to allow a portion of the liquid coolant flowing upwardly through the bundle to pass through the rods. In some cases, the water rods transition at both ends to a larger diameter center section which extends for most of the axial lengths of the rods. It has been attempted to combine side entry holes and a metering device within the lower diameter transition, but this arrangement has not always permitted the accurate regulation of coolant within the water rod, and because of the location of the metering device, is relatively expensive to manufacture.
In all cases, it is desirable to accurately meter the flow through the water rods to the amount required to prevent boiling within the rods. If there is too little flow, the coolant will boil within the rods, thus negating the purpose of the rods which is to distribute non-boiling water throughout the bundle length. Too much flow, on the other hand, starves the region around the fuel rods, outside the water rods.